DC-DC converters for supplying a stable DC power to various types of electronic devices can be classified roughly into a chopper type DC-DC converter without a synchronous rectifier and a synchronous-rectifier type DC-DC converter with a synchronous rectifier.
In a chopper type DC-DC converter, a DC power on a primary side is intermittently supplied to a coil by operation of a switching element, and a stepped up or down DC output is obtained on a secondary side by using magnetic energy stored in the coil.
FIG. 9 is an example of a chopper type DC-DC converter and illustrates a diagram of a DC-DC converter that is disclosed in JP-A-2001-161068 and has a supply power limiting function.
The chopper type DC-DC converter shown in FIG. 9 mainly includes a switching transistor (switching element) 21, a smoothing coil 22, a smoothing capacitor 23, and a rectifying diode 24. Further, the DC-DC converter includes a PWM comparator 20 and a pulse width limiting voltage setting section 25. An output voltage of the pulse width limiting voltage setting section 25 changes depending on high/low of an input power supply voltage.
In the DC-DC converter shown in FIG. 9, the output voltage of the pulse width limiting voltage setting section 25 becomes high when the input power supply voltage becomes high, and the output voltage of the pulse width limiting voltage setting section 25 becomes low when the input power supply voltage becomes low. In this way, the output voltage of the pulse width limiting voltage setting section 25 serves as a function to limit a maximum value of a time period (pulse width of a switching pulse), where the PWM comparator 20 outputs a voltage during one cycle of a triangle wave output voltage, to or below a limit width that changes depending on the input power supply voltage.
In contrast, a synchronous-rectifier type DC-DC converter employs a synchronous rectifier instead of a rectifying diode of a chopper type DC-DC converter.
FIG. 10 is an example of a synchronous-rectifier type DC-DC converter and illustrates a circuit diagram of a DC-DC converter disclosed in JP-A-2007-151271.
The DC-DC converter shown in FIG. 10 includes a chopper circuit 1, an error amplifier (output error detection circuit) 2, a control drive circuit (control section) 3, and a voltage detection circuit (voltage detection section) 4.
The chopper circuit 1 includes a main switch element (first switch) 11 that performs an ON/OFF operation in response to a signal inputted to a control terminal, an inductor 12 that repeats charge and discharge of magnetic energy according to the ON/OFF operation of the main switch element 11, a smoothing capacitor 13 that smoothes an electric current flowing through the inductor 12, and a rectifying switch element (second switch) 14 that performs an ON/OFF operation complementary to the ON/OFF operation of the main switch element 11. An input DC voltage Vi is applied to the main switch element 11, and an output DC voltage Vo is outputted from the smoothing capacitor 13 to a load 15. When switching, there is a dead-time during which neither the main switch element 11 nor the rectifying switch element 14 is ON. An antiparallel diode is added to each of the main switch element 11 of p-channel and the rectifying switch element 14 of n-channel. The diode is a body diode that is formed at the same time as a MOS transistor is formed to a semiconductor substrate. A typical synchronous-rectifier type DC-DC converter is configured such that the body diode does not operate.
In the DC-DC converter shown in FIG. 10, the error amplifier (output error detection circuit) 2 detects the output DC voltage Vo and generates an error signal Ve by amplifying a difference between the output DC voltage Vo and a target value. The voltage detection circuit 4 detects the error signal Ve outputted by the error amplifier 2 and outputs a detection signal depending on a difference between the error signal Ve and a target detection level. The control drive circuit 3 includes a current detection circuit 36 that generates a current detection signal Vc depending on an electric current of the main switch element 11, a comparison circuit 32, an oscillation circuit 31 that generates a clock signal Vck having a predetermined switching frequency and pulse width, a NOR gate 35, a latch circuit 34 that generates a drive signal DR, a drive circuit 33 that generates a first drive signal Vg1 for driving the main switch element 11, and a dead-time adjustment circuit 37 that generates a signal Vg20 and a second drive signal Vg2. The signal Vg20 has a logic level opposite to that of the first drive signal Vg1 and has a dead-time set to a minimum value. The signal Vg2 has a dead-time that is adjusted depending on the signal Vg20 and the detection signal from the voltage detection circuit 4. The dead-time of the signal Vg2 is a period of time from when the main switch element is turned OFF to when the rectifying switch element 14 is turned ON.
In comparison with the chopper type DC-DC converter shown in FIG. 9, the synchronous-rectifier type DC-DC converter shown in FIG. 10 can generally achieve a high conversion efficiency by shortening the dead-time during which neither the high-side main switch element 11 nor the low-side rectifying switch element 14 is ON.
Further, the DC-DC converter shown in FIG. 10 is configured by the dead-time adjustment circuit 37 in such a manner that a dead-time of the rectifying switch element 14 becomes longer as the output DC voltage Vo becomes larger. As a result, an increase in the output DC voltage Vo can be reduced.
However, even in the DC-DC converter shown in FIG. 10, for example, like in the case of a load dump that is a surge (abnormal voltage) specific to a vehicle, when a sudden increase in an input voltage occurs or a high input voltage is continuously applied for a long time, it is difficult to control the dead-time because operation characteristics of the switch elements 11, 14 change. Therefore, there may be a possibility that the two switch elements 11, 14 will be simultaneously turned ON and broken by a flow-through current.